1. Field of the Invention
The invention relates to a fuel cell and an element for the fuel cell and, more particularly, to a fuel cell to which a mixed gas containing a fuel gas, such as methane, and oxygen is fed and to a multi-element stack for the fuel cell.
2. Description of the Related Art
A fuel cell can be expected to have a high efficiency of power generation compared to power generation in a thermal power plant, and is currently being studied by many researchers.
As shown in FIG. 6, such a conventional fuel cell is provided with an element 106 for the fuel cell, which element uses, as a solid electrolyte layer 100 of an oxygen ion conduction type, a fired body made of stabilized zirconia to which yttria (Y2O3) is added, the solid electrolyte layer 100 having one side on which a cathode layer 102 is formed, and another side on which an anode layer 104 is formed. Oxygen or an oxygen-containing gas is fed to the side of cathode layer 102 of the fuel cell element 106, and a fuel gas, such as methane, is fed to the side of anode layer 104.
The oxygen (O2) fed to the side of cathode layer 102 of the fuel cell element 106 is ionized into oxygen ions (O2xe2x88x92) at the boundary between the cathode layer 102 and the solid electrolyte layer 100, and the oxygen ions are conducted to the anode layer 104 by the electrolyte layer 100. The oxygen ions conducted to the anode layer 104 react with the methane (CH4) gas fed to the side of anode layer 104, to thereby form water (H2O), carbon dioxide (CO2), hydrogen (H2), and carbon monoxide (CO). During the reaction, the oxygen ions release electrons, resulting in a potential difference between the cathode layer 102 and the anode layer 104. Accordingly, by establishing an electrical connection between the cathode layer 102 and the anode layer 104 by a lead wire 108, the electrons of the anode layer 104 pass in the direction toward the cathode layer 102 (the direction of the arrow in the drawing) through the lead wire 108, and electricity can be produced by the fuel cell.
The fuel cell shown in FIG. 6 is operated at a temperature of about 1000xc2x0 C. At such a high temperature, the side of cathode layer 102 of the fuel cell is exposed to an oxidizing atmosphere, and the side of anode layer 104 is exposed to a reducing atmosphere. Consequently, it has been difficult to enhance the durability of the element 106.
It is reported, in Science, vol. 288, pp2031-2033 (2000), that, as shown in FIG. 7, even when a fuel cell element 206 made up of a solid electrolyte layer 200, and a cathode layer 202 and an anode layer 204 respectively formed on one side and another side of the electrolyte layer 200, is placed in a mixed gas atmosphere of methane and oxygen, the fuel cell element 206 develops an electromotive force.
By placing the element 206 in a mixed gas atmosphere, as above, the element 206 can be enveloped as a whole in substantially the same atmosphere, and can have improved durability compared to the element 106 shown in FIG. 6, in which the respective sides of the element 106 are exposed to atmospheres different from each other.
Nevertheless, the fuel cell shown in FIG. 7 has only a single element for the fuel cell (or fuel cell element) 206 contained in a container 210, so that the voltage which can be taken out of the fuel cell is low.
To obtain a desired level of voltage from a fuel cell using the type of an element illustrated in FIG. 7, a multi-element stack comprising a plurality of fuel cell elements 206, as shown in FIG. 8, is used. In the multi-element stack, the fuel cell elements 206 are separated from each other by a separator 300 provided with complex gas passages 302a, 302b, which make, in turn, the structure of the fuel cell complicated. In addition, in such a complicated fuel cell, it is difficult to make the respective members have equivalent properties, such as a coefficient of thermal expansion, at an operation temperature of the cell, and the members tend to be largely effected by a thermal stress.
It is an object of the invention to provide a fuel cell using a mixed gas containing a fuel gas, such as methane, and oxygen, which gas a simple structure and from which a desired level of voltage can be obtained, and a multi-element stack for the fuel cell.
To this end, the inventors found that a fuel cell using a multi-element stack having a simple structure, in which a plurality of fuel cell elements, such as those shown in FIG. 7, are stacked without the use of separators, can have a simple structure, and can provide a desired level of voltage.
Thus, the invention resides in a fuel cell comprising a container having a gas inlet and a gas outlet, and a multi-element stack contained in the container and made up of two or more elements for the fuel cell, the element comprising an electrolyte layer, a cathode layer, and an anode layer, with the electrolyte layer being interposed between the cathode and anode layers, and a mixed gas of a fuel gas and an oxygen-containing gas being fed to the fuel cell from the gas inlet, wherein the multi-element stack is formed of the elements stacked in such a manner that the cathode layer of one element is in direct contact to the anode layer of another element, and each of the electrolyte, cathode, and anode layers has a passage through which the mixed gas passes.
The invention also resides in a multi-element stack for a fuel cell to which a mixed gas of a fuel as and an oxygen-containing gas is fed, the multi-element stack being made up of two or more elements, the element comprising an electrolyte layer, a cathode layer, and an anode layer, with the electrolyte layer being interposed between the cathode and anode layers, wherein the multi-element stack is formed of the elements stacked in such a manner that the cathode layer of one element is in direct contact with the anode layer of another element, and each of the electrolyte, cathode, and anode layers has a passage through which the mixed gas passes.
In the fuel cell according to the invention, at least a part of the outer surfaces of the multi-element stack is in intimate contact with the inner surface of the container, and/or the gap between the outer surface of the multi-element stack and the inner surface of the container is sealed. This makes it possible to allow the mixed gas fed to the cell to certainly pass through the multi-element stack without bypassing it, leading to the lowered running cost of the fuel cell.
In the invention, at least one of the electrolyte, cathode, and anode layers may be porous. It is preferred that the cathode and anode layers are porous, and the electrolyte layer is solid, and has a hole piercing through it from one side to the other side of the electrolyte layer, whereby the mixed gas can pass from the gas inlet to the gas outlet through the pores in the cathode and anode layers and the hole in the electrolyte layer. In this case, the cathode and anode layers may further have a shaped passage for the mixed gas, the shaped passage having a size larger than the diameter of the pores in the cathode and anode layers.
It is also preferred that the electrolyte, cathode, and anode layers are porous, so that the mixed gas can pass from the gas inlet to the gas outlet through the pores in these layers.
Preferably, the porous layers have an open porosity of equal to or greater than 20%, more preferably 30 to 70%, and most preferably 40 to 50%.
Preferably, the electrolyte layer is formed of a zirconium oxide (zirconia) which is partially stabilized by an element of group III of the periodic table, such as yttrium (Y) or scandium (Sc), or a cerium oxide which is doped with an element such as lanthanide, for example, samarium (Sm) or gadolinium (Gd).
Also preferably, the cathode layer is formed of a manganite, gallate or cobaltite compound of lanthanum to which an element of group III of the periodic table, such as strontium (Sr), is added.
Also preferably, the anode layer is formed of a body supporting nickel cermet or platinum to which 10 to 30% by weight of an electrolyte forming the solid electrolyte layer 10a, 10b is added.
Preferably, the mixed gas is selected from hydrogen, methane, ethane, propane, butane, and mixtures thereof, and the oxygen-containing gas is oxygen or air.